This invention relates to membrane modules used to transfer a gas to or from a liquid and to a process using a membrane supported biofilm for treating wastewater to remove one or more of nitrogen, phosphorous, BOD and COD.
Transferring gases to or from a liquid is most commonly practiced by providing a bubble diffuser in the liquid. As bubbles rise through the liquid, gases move across the boundary of the bubble driven by the relative partial pressures of the gas in the bubble and in the liquid. Such a process has serious drawbacks including high energy costs, difficulty in independently controlling mixing of the liquid, foaming on the liquid surface and lack of control over the gas released by the bubbles as they break at the liquid surface. Gas permeable membrane modules provide an alternate means for transferring a gas to or from a liquid and have been used in various reactor designs. Some examples are described below.
U.S. Pat. No. 4,181,604 (issued to Onishi et al. on Jan. 1, 1980), describes a module having several loops of hollow fibre membranes connected at both ends to a pipe at the bottom of a tank containing wastewater. The pipe carries a gas containing oxygen to the lumens of the membranes. Oxygen flows through the membranes to the wastewater and to an aerobic biofilm growing on the outer surface of the membranes. In U.S. Pat. No. 4,746,435 (issued to Onishi et al. on May 24, 1988), the same apparatus is used but the amount of oxygen containing gas is controlled to produce a biofilm having aerobic zones and anaerobic zones.
U.S. Pat. No. 4,416,993 (issued to McKeown on Nov. 22, 1983), describes a membrane module in the form of a hollow plate. The plates are made of a rigid frame wrapped in a porous xe2x80x9cnettingxe2x80x9d made of PTFE laminated to a woven nylon fabric. The plates are attached to an overlapping strip which has an inlet port and an outlet port.
In xe2x80x9cBubble-Free Aeration Using Membranes: Mass Transfer Analysisxe2x80x9d (Journal of Membrane Science, 47 (1989) 91-106) and xe2x80x9cBubble-Free Aeration Using Membranes: Process Analysisxe2x80x9d (Journal Water Pollution Control Federation, 1988, Volume 60, Number 11, 1986-1992), Cxc3x4txc3xa9 et al. describe the use of silicone rubber tubes to transfer oxygen to water without creating bubbles in the water. The apparatus for these studies includes a module having vertically oriented tubes suspended between an inlet header and an outlet header. The module is immersed in a tank containing water recirculated by a pump to provide a horizontal current in the tank.
U.S. Pat. No. 5,116,506 (issued to Williamson et al. on May 26, 1992) describes a reactor having a gas permeable membrane dividing the reactor into a gas compartment and a liquid compartment. The gas compartment is provided with oxygen and methane which diffuse through the membrane to support a biofilm layer in the liquid compartment. The membrane is made of a teflon and nylon laminate commonly known as Gore-tex (TM). In one embodiment, the membrane divides the reactor into lower and upper portions. In another embodiment, the gas compartment rotates within the liquid compartment.
In xe2x80x9cStudies of a Membrane Aerated Bioreactor for Wastewater Treatmentxe2x80x9d (MBR 2xe2x80x94Jun. 2, 1999, Cranfield University), Semmens et al. describe a membrane module having microporous polypropylene hollow fibres stitched together to form a fabric. The fabric is mounted between a gas inlet header and a gas outlet header such that the fibres are oriented horizontally. The module is immersed in water in an open reactor with water recirculated by a pump to provide a horizontal current in the reactor.
Despite the variety of designs available, gas transfer membranes have not achieved widespread commercial success. Common criticisms of modules or reactors include (a) that membrane materials lack sufficient strength to be durable in hostile environments (b) that membrane surface area is inadequate, particularly for a tank of a fixed and pre-selected size, (c) that excessive movement of liquid is required which is costly to implement in large systems, (d) that biofilm growth on the membranes is difficult to prevent or maintain at a controlled thickness and (e) that even small leaks or defects in the membranes cause a significant loss of system capacity.
Gas transfer is used for a number of processes, one of which is wastewater treatment. Discharging wastewater containing large amounts of carbon (BOD or COD), nitrogen and phosphorous into a natural body of water causes eutrophication, algae blooms, pollution and health problems. Various processes have been developed to treat wastewater to remove some or all of the carbon, nitrogen and phosphorous, some of which will be summarized below.
Activated Sludge with Chemical Phosphorous Removal
In a typical activated sludge process, wastewater flows in series through an anoxic reactor, an aerobic reactor and a clarifier. Effluent from the clarifier is released to the environment. Activated sludge from the bottom of the clarifier is partially recycled to the anoxic reactor and partially wasted. Significant removal of nitrogen requires a significant rate of recycle to alternately nitrify and denitrify the wastewater.
Phosphorous is removed by dosing soluble metal salts, such as ferric chloride or aluminum sulphate, at one or more points in the process into the aerobic reactor to precipitate phosphate metal salts. The waste water, however, contains many different ions which create undesirable side reactions. As a result, and particularly where very low effluent total phosphorus levels are required, precipitating phosphorous may require the addition of 2-6 times the stoichiometric amount of the metal salt. Accordingly, these processes result in high chemical costs, high sludge production, and a high level of metallic impurities in the sludge.
Activated Sludge with Biological Phosphorous Removal
Activated sludge techniques can also be modified to use microorganisms to store the phosphates. For example, U.S. Pat. No. 4,867,883 discusses a process which attempts to encourage the selection and growth of Bio-P organisms which uptake phosphorus in excess of the amount normally needed for cell growth. Generally, the process consists of an anaerobic zone, an anoxic zone, an aerobic zone, and a clarifier. In the anaerobic zone, soluble BOD is assimilated and stored by the Bio-P organisms and phosphorus is released. Subsequently, in the anoxic and aerobic zones, the stored BOD is depleted and soluble phosphorous is taken-up in excess and stored as polyphosphates by the Bio-P organisms. In the clarifier, sludge containing phosphates settles out of the effluent. There is a denitrified recycle from the anoxic zone to the anaerobic zone, a nitrified recycle from the aerobic zone to the anoxic zone, and an activated sludge recycle from the clarifier to the anoxic zone. The sludge recycle is done in multiple phases to ensure that nitrates are not recycled to the anaerobic zone, which would limit phosphorous release. The biological mechanism by which bacteria release phosphorous in the anaerobic section involves the uptake of easily assimilated organic compounds such as volatile fatty acids (VFA). Depending on the level of VFA in the raw wastewater, an extra anaerobic section may be added at the head of the process.
One problem with this process is that the settling characteristics of the sludge in the clarifier impose significant design limitations. For example, the process cannot operate at very high process solids levels or high sludge retention times, particularly when high removal rates of both nitrogen and phosphorous are required. As a result, the system is generally considered to be inefficient and there is a high generation rate of waste sludge. In some cases, sand filters are added to the tail of the process to help remove solids carryover from an overloaded clarifier and reduce the amount of phosphorous in the effluent.
Another problem with this process is that there is a buildup of phosphates in the system. The waste activated sludge contains Bio-P organisms rich in phosphorous. When the organisms in the waste activated sludge are digested, they release phosphorus which is typically returned back to the process in the form of digester supernatant. Consequently, this reduces the efficiency of phosphorus removal in the process and results in higher levels of phosphorus in the effluent. A partial solution to this problem is to employ a side stream process called xe2x80x98Phos-Pho Stripxe2x80x99 as described in U.S. Pat. No. 3,654,147. In this process, activated sludge passes from the clarifier to a phosphorus stripper. In the stripper, phosphorus is released into the filtrate stream by either: creating anaerobic conditions; adjusting the pH; or extended aeration. The resulting phosphate-rich filtrate stream passes to a chemical precipitator. The phosphate-free effluent stream is added to the main effluent stream, the waste stream from the precipitator containing the phosphates is discarded, and the phosphate-depleted activated sludge is returned to the main process.
Membrane Bioreactor with Chemical Precipitation
A membrane bioreactor can be combined with chemical precipitation techniques. In a simple example, precipitating chemicals are added to an aerobic tank containing or connected to a membrane filter. As above, however, dosages of precipitating chemicals substantially in excess of the stoichiometric amount of phosphates are required to achieve low levels of phosphates in the effluent. This results in excessive sludge generation and the presence of metallic precipitates which increase the rate of membrane fouling or force the operator to operate the system at an inefficient low sludge retention time.
Membrane Supported Biofilm
U.S. Pat. No. 4,181,604 describes a module having several loops of hollow fibre membranes connected at both ends to a pipe at the bottom of a tank containing wastewater. The pipe carries a gas containing oxygen to the lumens of the membranes through which the gas is supplied to the wastewater and to an aerobic biofilm growing on the outer surface of the membranes. In U.S. Pat. No. 4,746,435, the same apparatus is used but the amount of oxygen containing gas supplied is controlled to produce a biofilm having aerobic zones and anaerobic zones and 1 to 7 ppm of oxygen in the waste water. This process provides simultaneous nitrification and denitrification without sludge recirculation but no phosphorous removal.
U.S. Pat. No. 5,116,506 describes a reactor having an oxygen containing gas permeable membrane separating a reactor into a liquid compartment and a gas compartment. The liquid compartment contains wastewater. The gas compartment is provided with oxygen which diffuses through the membrane to support a biofilm layer. The biofilm layer has two parts, an aerobic layer adjacent the membrane and an anaerobic layer adjacent the wastewater. This process also provides simultaneous nitrification and denitrification but again no phosphorous removal.
It is an object of the present invention to provide a membrane module for transferring a gas to or from a liquid. Such modules can be used, for example, in supporting and providing oxygen to a biofilm, in water degassing, in humidification, in pervaporation and to clean air. An object of the present invention is to provide a process for treating wastewater to produce an effluent with reduced concentrations of one or more of nitrogen, phosphorous and carbon (BOD or COD). These objects are met by the combination of features, steps or both found in the claims. The following summary may not describe all necessary features of the invention which may reside in a sub-combination of the following features or in a combination with features described in other parts of this document.
In one aspect, the invention provides an apparatus for transferring a gas to or from a liquid having a flexible and gas diffusive but liquid water impermeable membrane and a flexible spacer open to gas flow. The spacer and the membrane together form a planar element with the membrane enclosing an inner space containing the spacer. One or more conduits are provided for transferring gas between the inner space and the atmosphere or another location outside of the water and the inner space. One or more tensile members or weights non-rigidly restrain the planar element in a selected position in a selected reactor. Gases that may be transferred include oxygen, nitrogen, volatile organic compounds, hydrogen, and water vapour.
In another aspect, the invention provides a module for transferring a gas to or from a liquid having a plurality of the apparatus described above and a gas manifold. The second ends of the gas inlet conduits are connected in fluid communication with the manifold to admit gas to the planar elements. The manifold is mounted above the water surface of a reactor while the planar elements are located below the water surface of the reactor. The reactor has a tank having a generally straight flow path covering a substantial portion of the tank between an inlet and an outlet. The planar elements are restrained in positions in the reactor in which they are generally parallel to the flow path. In a wastewater treatment applications, the reactor has a source of agitation for agitating the planar elements to release accumulated biofilm from time to time.
In another aspect, the invention is directed at a process for transferring a gas to or from a liquid comprising the steps of (a) immersing one or more of the planar elements described above in the liquid and (b) supplying a gas to the planar elements at a pressure which does not create bubbles in the liquid, the gas leaving the planar elements by diffusion or by forced circulation using a pump. For some embodiments, the pressure of the gas is preferably also less than the pressure of the wastewater against the planar elements.
In another aspect, the invention provides a hybrid wastewater treatment reactor combining a membrane supported biofilm and suspended growth biomass. The reactor has a first section containing a plurality of gas transfer membrane modules connected to an oxygen source and a second section having an oxygen source operable to create aerobic conditions in the second section. In the first section, the supply of oxygen to the membrane modules is controlled to cultivate a biofilm on the surface of the membranes having aerobic and anoxic zones and to facilitate cultivation of an anaerobic mixed liquor in the first section generally. In the second section, the diffusers and oxygen source facilitate cultivation of an aerobic mixed liquor. Wastewater enters the reactor through an inlet to the first section and flows through the reactor so as to be treated in the anaerobic section, in the aerobic section and by contact with the biofilm before leaving the reactor through a solid/liquid separator downstream of the second section. A portion of the settled sludge at the bottom of the clarifier is recycled to the first section.
Biological digestion of BOD, COD, nitrogen and phosphorous are achieved as summarized below:
Rough removal of BOD or COD and nitrogen occur in the biofilm.
Polishing denitrification and sludge reduction occur in the anaerobic mixed liquor.
Volatile fatty acids (VFA) are assimilated and phosphorous is released in the anaerobic mixed liquor.
polishing COD and BOD removal, polishing nitrification and biological phosphorous uptake occur in the aerobic mixed liquor.
phosphorous is extracted as excess biomass by wasting a portion of the sludge settled in the clarifier.
In another aspect, the invention provides a modified reactor in which phosphorous is also extracted as a chemical precipitate. The anaerobic mixed liquor is most often quiescent allowing partial sedimentation of the anaerobic mixed liquor which produces a phosphorous rich solution near its surface. Alternatively, a portion of the anaerobic mixed liquor is treated in a solid-liquid separation device to produce a phosphorous rich solution. The phosphorous rich solution is treated in a precipitation branch having a source of phosphorous precipitating agents such as metal salts and a precipitate separation device such as a clarifier or hydrocyclone.